The company that took over the running of the leisure centres from the city council almost 18 months ago calculated that the Unite members who backed industrial action represented 13% of the total workforce.

A statement from London-based Greenwich Leisure Limited (GLL) said: "Support for the strike was weak.

"Unite represent 130 of a total of 500-plus staff working for GLL across the city of Belfast.

"Sixty-six Unite staff voted to strike in the ballot, representing just 13% of our total workforce.

"On the day, 120 staff turned up for work and nearly 1,000 customers used our facilities, including two school groups totalling nearly 100 children. All services were provided in a fully safe manner that complies with all legislation."

However, Unite regional official Davy Edmont challenged the company's figures.

"There are around 350 full-time employees, and Unite represents around 140 of them, which is more than 40%," he said.

"Of the 140 who were balloted, there were 68 ballots returned, and that is not 13% of the total workforce. From our point of view, I think our first day of action was a great success."

There was also a warning that the dispute could escalate, with representatives of the Northern Ireland Public Service Alliance (Nipsa) due to meet GLL later today.

No one from Nipsa was available for comment yesterday, but a statement from GLL, which runs the centres under the brand name Better, said: "There is a meeting with Nipsa, and GLL would hope to resolve any issues that Nipsa may have to avoid escalation."

However, Mr Edmont insisted there was support for the ramping-up of industrial action

"My understanding is that Nipsa will also be balloting its members, and together we account for more than 90% of the staff," he said.

Unite also claimed that a number of swim clubs that use the centres refused to cross picket lines, along with many members of the public.

The strike came after disputes over staffing levels, safety provision, pensions, how centres are being run and a requirement for unaccompanied children to be able to swim 25 metres.